The mills of the North American carpet industry and their yarn suppliers handle over 200 million BCF yarn packages per year, consisting of yarn wound around heavy paper, plastic or composite rolls, called “tube cores.” Each of these BCF packages normally contain from about 8 to 20 pounds of yarn, depending on the bulk of the yarn, where bulk is a measure of the space taken up by a given weight of yarn. The bulkier the yarn, the less weight the package generally contains. The carpet industry often uses tube cores, sometime multiple times, depending on the yarn type and the processes involved. However, the expense of cores is still a substantial cost item. Furthermore, it is important to understand that cost is incurred each time a package is handled, both in terms of manpower and from risk of damage to both the yarn and the tube core.
The physical dimensions of the BCF yarn package are not easily changed. The size and makeup of the standard BCF package is set by several factors, including the limitations of existing spinning, winding, and unwinding processes and equipment. For example, tube core diameter must be large enough to permit smooth unwinding, while it must also be strong enough to permit winding at high speed. The overall diameter of the BCF yarn package is also restricted, in one case by the standard twister bucket diameter, into which the package must fit. The stroke, or width of the yarn on the tube core is also set in accordance with existing equipment size and process limitations, including unwinding efficiency.
Several methods of increasing yarn package density have been employed. These include: tighter winding around the tube core and tighter yarn packing with overlapping loops. These methods, however, have their unique drawbacks, which include difficulty removing the yarn; loss in bulk property; decrease in package stability; and yarn falling off the core ends. To avoid the above problems, precision winding and random winding methods are used.
Precision Winding is typically used for textile yarns, which are fine denier and flat, meaning they are not bulk textured and so contain almost no “bulk” property. These yarns are typically textured in secondary steps, and the smoothness and uniformity of unwinding is most important to subsequent process productivity. Wound packages of textile yarn are also typically finer denier. Owing to these factors, textile yarn packages typically contain a very much greater length of yarn than BCF packages and both wind and unwind at higher speeds than is presently typical for BCF. A precision winding control method and winding profile designed to avoid ribbon formation is provided in U.S. Pat. No. 5,056,724 to Prodi and Albonetti, where operating limits are established, for example at the ribbon formation winding ratios, and then avoided. Another profile described in U.S. Pat. No. 6,311,920 to Jennings et al is designed to avoid package irregularities by winding adjacent to integral and sub-integral winding ratios and imposing a consistent offset from each winding ratio throughout the package.
For BCF yarn, it is customary to use a random wind profile in which a constant helix angle/wind ratio is maintained through adjusting spindle speed and traverse guide speed. The result of this approach is a random yarn lay pattern on the BCF package with varied spacing between the yarn threads throughout the package. This tends to provide a stable package with few winding problems, and it avoids the “ribbon” problem described above. A somewhat more advanced example of this approach maintains a constant crossing angle as yarn layers overlap on the package, such as is disclosed by Haak in U.S. Pat. No. 5,740,981, applied to both spindle driven and friction drive winding systems. Randomly wound packages vary greatly in packing density, depending especially on yarn bulk, where yarns of higher bulk make lighter weight packages.